Many studies have so far been made with respect to reflector antenna systems. For example, as to beam scanning characteristics of Gregorian antenna systems, a paper entitled "Beam Scanning Characteristics of Offset Gregorian Antennas" was made public by M. Akagawa et al in IEEE International Symposium Digest--Antennas & Prop in 1979. Further, as to the technique of eliminating cross polarization in offset dual reflector antenna systems, a report entitled "Elimination of Cross Polarization in Offset Dual-Reflector Antennas" is made by H. Tanaka et al in Trans IECE, Japan, '75/12 Vol. 58-B No. 12.
In addition to the antenna disclosed in the above literature, there are also known as conventional examples respectively shown in FIGS. 1, 2 and 3 as will be described below.
FIG. 1 schematically shows a partial section of conventional offset Cassegrain antennas used as dual reflector multi-beam antenna systems.
For the simplification of explanation, it is assumed that, as shown in FIG. 1, a main reflector 1 comprises a rotary parabolic mirror having a focus F.sub.1 and a subreflector 2 comprises a rotary hyperbolic mirror having focuses F.sub.o and F.sub.1. It is further, assumed that on the basis of a reference mirror which is determined geometro-optically when a primary receiver 3a is disposed on the focus F.sub.o, primary radiators 3b and 3c are respectively disposed on points F.sub.o ' and F.sub.o " in the vicinity of F.sub.o to scan electromagnetic wave beams. In this case it is apparent from the geometro-optically standpoint that one or both of the main reflector and the subreflector are required to be larger than the reference mirror. Therefore, the following description will be based on the assumption that the main reflector 1 is fixed to the reference mirror and only the subreflector 2 is made larger than the reference mirror.
As shown, an electromagnetic wave beam incident from the front, namely, from the right in the figure, is received by the primary receiver 3a located on the focus F.sub.o via the reflector 1 and the subreflector 2. Likewise, other electromagnetic wave beams incident from below and above the said electromagnetic wave beam are received respectively by the primary receivers 3b and 3c located on F.sub.o ' and F.sub.o " via main reflector 1 and subreflector 2. In the antenna system having such an arrangement, in case the beam scanning angle is small, it is possible to minimize the generation of cross polarization component by selecting the arrangement of the reflector 1 and the subreflector 2 so as to satisfy the conditions for eliminating cross polarization, and it is possible to enlarge the focus length F to diameter D ratio (F/D) of offset parabola equivalent to the antenna system, as shown in FIG. 1, so that the characteristic deteriorations induced by beam scanning, such as a decrease in gain and increase in side lobes can be minimized.
However, in case the beam scanning angle is large, as shown in FIG. 3a, there may occur the case where part of an electromagnetic wave reflected by the reflector 1 passes without striking against the subreflector 2.
In view of the problem just mentioned, there has been proposed an antenna system whose arrangement is sectionally shown in FIG. 2(b). As shown in the figure, the subreflector 2 has a fairly large diameter and its reflective portions relating to the beaming directions are different from each other, thus deteriorating the utilization efficiency of the subreflector 2. Further, the positions F.sub.o, F.sub.o ' and F.sub.o " respectively of the radiators 3a, 3b and 3c disposed relating to the beaming directions are spaced apart from one another by a substantial distance, so that the configuration space of the primary radiators becomes fairly large.
FIG. 3 is a sectional view schematically showing the construction of a conventional offset Gregorian antenna used as a dual reflector multi-beam antenna.
As will be apparent from FIG. 3, the antenna system illustrated therein is advantageous over the antenna systems shown in FIGS. 2a and 2b in that the antenna construction can be rendered compact even in case the beam scanning angle is large. However, also in this case, the configuration space of the primary radiators becomes large because the positions F.sub.o, F.sub.o ' and F.sub.o " of the radiators 3a, 3b and 3c respectively are dispersed. Further, the curvature of the subreflector 2 becomes large, so even if the arrangement of the main and subreflectors 1 and 2 is so selected as to satisfy the conditions for eliminating cross polarization, its characteristics in beam scanning are fairly deteriorated and decrease in gain is conspicuous.